1. Field of the Invention
This invention relates to a solar cell power system, and in particular, to a solar cell power system for satellites or the like which is of the type adapted to supply power from a solar cell to a load during the sunlight while stabilizing the voltage by means of a shunt device, and also to supply power to the load through discharge from a storage battery in the eclipse time, the system having a solar array bus lockup cancelling mechanism which serves to cancel a stage in which the voltage of the solar cell power system is fixed to that of the storage battery during the sunlight and a battery continues to discharge for a long time even when the power supply to the load can be met solely with the power generated by the solar cell (The state will be hereinafter referred to as "solar array bus lockup") so that the system is restored to a state in which the voltage is regulated by the shunt device.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the construction of a conventional solar cell power system, which includes a solar cell 1, a storage battery 2, a first diode 3 whose anode is connected to the output terminal of the solar cell 1, and a power bus 4 which is connected to the cathode of the first diode 3.
The system further includes a second diode 5 whose cathode is connected to the cathode of the first diode 3 through the power bus 4; a shunt device 6, which is connected between the anode of the first diode 3 and a return line (hereinafter referred to simply as RTN) in parallel with the solar cell 1, and which is adapted to consume any surplus power generated by the solar cell 1; a battery charger 7, which is connected to the power bus 4 in parallel with the second diode 5 and which serves to charge the storage battery 2 during the sunlight; a capacitor bank 8 which is connected between the power bus 4 and the RTN; and a load 9 which is connected to the power bus 4 and the RTN, and whose magnitude is set at the ground station with a command CE. In addition, the system includes a solar array current monitor 10, which is adapted to detect the level of the current flowing through the first diode 3 into the power bus 4 and convert it to a telemetry signal I.sub.SATLM before its transmittal; a load current monitor 11, which is adapted to detect the level of the current flowing through the power bus 4 into the load 9 and convert it to a telemetry signal I.sub.LTLM before its transmittal; a charge/discharge current monitor 12, which is connected between the anode of the second diode 5 and the storage battery 2, and which is adapted to detect the level of the charge/discharge current of the storage battery 2 and convert it to a telemetry signal I.sub.CTLM before its transmittal; and a bus voltage monitor 13 which is adapted to detect the voltage of the capacitor bank 8 and convert it to a telemetry signal V.sub.BUSTLM before its transmittal.
The operation of this conventional power system will now be described in detail.
During the sunlight, the power generated by the solar cell 1 is supplied to the load 9 through the first diode 3. Any surplus power that results when the power generated by the solar cell 1 exceeds the power consumed by the load 9 is partly converted by the battery charger 7 to charge the storage battery 2, and the rest is consumed by the shunt device 6.
In this process, the voltage of the capacitor bank 8 (hereinafter referred to as bus voltage) is regulated to a value V.sub.SHNT. The value of the bus voltage V.sub.SHNT is generally set to be constantly higher than that of the voltage V.sub.BAT of the storage battery 2.
FIG. 2 shows the relationship between the power generated by the solar cell 1 and the load power consumed by the load 9. The power generated by the solar cell 1 is represented by the I.sub.S -V.sub.S curve of FIG. 2. The load 9 appears to be a constant-power load since it generally contains a built-in DC/DC converter and consumes power while converting the bus voltage to an appropriate constant voltage. Accordingly, the relationship between the load voltage and the load current can be represented by the curve P--P' shown in FIG. 2.
As stated above, the shunt device 6 consumes any surplus power generated by the solar cell 1 so that the bus voltage may be regulated to V.sub.SHNT. As a result, the intersection point A of the straight line M--M' and the constant power line P--P' of FIG. 2 represents the power operating point.
In the eclipse time, power generation by the solar cell 1 is stopped, so that power is supplied to the load 9 through discharge of the storage battery 2 through the intermediation of the second diode 5. The bus voltage at this time is equal to the discharge voltage V.sub.BAT of the storage battery 2.
In order to monitor the operating condition of the power system, the solar array current monitor 10, the load current monitor 11, the charge/discharge current monitor 12, and the bus voltage monitor 13 detect the current or voltage level at different parts of the system, as stated above, and convert them to telemetry signals, which are transmitted to the ground station.
FIG. 3 shows the transition of the power operating point when load fluctuation occurs during a period of sunlight. In FIG. 3, the curve I.sub.S -V.sub.S represents the current/voltage characteristic of the power generated by the solar cell 1; V.sub.BAT represents the bus voltage value when the storage battery 2 is discharging; and V.sub.SHNT represents the value of the regulated bus voltage obtained by the solar-cell surplus-power control effected by the shunt device 6.
The case considered will be that where the power consumption by the load 9 fluctuates when the power consumed by the load 9 is P1 and the operating point is A. As long as the power consumption fluctuates within the range: V.sub.SHNT .times.I.sub.S, the power operating point lies in the straight line M--M' of FIG. 3.
If, however, the power consumption of the load 9 has exceeded the range of V.sub.SHNT .times.I.sub.S, increasing from P1 to P2, it exceeds the power generated by the solar cell 1, so that power compensation is effected by discharge from the storage batter 2. In this case, the power operating point moves from A to M, then to B.
Once it has moved to B, the operating point is not restored to A even if the power consumption of the load 9 is immediately reduced to P1 afterwards; it only moves to the point C. This brings about a state in which the bus voltage is fixed to the voltage of the storage battery 2, and the storage battery 2 continues to discharge, although the solar cell 1 is capable of generating all the power required by the load 9. This phenomenon is called solar array bus lockup.
To cancel solar array bus lockup, the power consumption of the load 9 is temporarily reduced to P3 or less by a command from the ground station, thus shifting the operating point in the order: C, D, E to A.
If solar array bus lockup is left unattended, the storage battery 2 will be allowed to discharge constantly, destroying the balance of power of the storage battery 2 between periods of sunlight and eclipse.
Conventionally, occurrence of solar array bus lockup has been detected in the following manner: first, discharge from the storage battery 2 is confirmed through the telemetry signal I.sub.CTLM of the charge/discharge current monitor 12. Then, a computer provided in the ground station performs a calculation using the following values: an engineering transformation I.sub.SA of the telemetry signal I.sub.SATLM of the solar array current monitor 10, an engineering transformation I.sub.L of the telemetry signal I.sub.LTLM of the load current monitor 11, and an engineering transformation V.sub.BUS of the telemetry signal V.sub.BUSTLM of the bus voltage monitor 13 as well as the regulated bus voltage value V.sub.SHNT obtained by the shunt device 6 for the purpose of checking whether the following inequality holds true or not: EQU V.sub.SHNT .times.I.sub.SA &gt;V.sub.BUS .times.I.sub.L ( 1)
With the conventional method, solar array bus lockup is judged to have been brought about if the inequality (1) holds true.
This method, however, can only be used when communication is always possible between the satellite and the ground station, as in the case of a geostationary satellite. A satellite in a relatively low earth orbit, is in a state for a considerable length of time when no communication with the ground station is possible, if solar array bus lockup occurs during such a period, the above method cannot be used until communication with the ground station again becomes possible. Accordingly, under these circumstances prompt cancellation of solar array bus lockup cannot be effected.
It is an object of this invention to provide a solar cell power system having a solar array bus lockup cancelling mechanism which is adapted to automatically detect the occurrence of solar array bus lockup and cancel it.